High efficiency class c multiplier



vouTAGE y 1951 J. c. FERGUSON v I 2,553,565

I HIGH EFFICIENCY CLASS C MULTIPLIER Filed Oct. v,v 1946 FIG.|

g OUTPUT SIGNAL SOURCE ADJUSTABLE PHASE SHIFTER FIG. 2

I STEADY COLLECTOR VOLTAGE\ 3| COLLECTOR VOLTAGE CURRENT CONDUCTING Y 30 PERlOD AST' STAGE VOLTAGE I STEADY LAST STAGE 34 INVENTOR 2 -l fl JOSEPH c. FERGUSON ATTORNEY i atented May 22, 1951 HIGH EFFICIENCY CLASS C MULTIPLIER Joseph C. Ferguson, Fort Wayne, Ind., assignor,

by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application October 7, 1946, Serial No. 701,841

8 Claims. (01. 179-171) This invention relates to electron multipliers and particularly to a high efficiency multiplier operated in the manner of a class C amplifier.

It has been suggested to operate an electron multiplier so that the current conducting period is appreciably less than 180 degrees of a cycle, that is, as a class C amplifier. The large gain in efficiency of a class C amplifier over class A or B operation is well known in the art. However, even a class C amplifier has an efiiciency which does not exceed approximately 85 per cent. At such a high efficiency level a further increase in efliciency even though small is very desirable because it is accompanied by a large saving in power and equipment. When the efficiency of an amplifier is thus increased, a tube having a certain rated plate dissipation may be operated at a higher power level resulting in an appreciably higher output signal.

In a conventional class C amplifier a certain loss in efficiency is caused by the fact that the minimum instantaneous plate voltage cannot be substantially less than the maximum grid swing. However, in an electron multiplier the control gridis usually provided between the cathode and the first multiplying stage so that the voltage of electrode need not exceed the value required ior collecting at the collector electrode all electrons which are liberated from the last multiplying stage. The voltage supplied to the last multiplying stage of a conventional multiplier has a fixed value so that the instantaneous voltage difference between the last multiplying stage and the collector electrode varies during the current conducting period thereof in view of the large voltage swing of the collector electrode.

During the major portion of the current conducting period this instantaneous voltage difference must therefore necessarily be larger than that required for collecting the secondary electrons. The result is that the electrons during that portion of the current conducting period have an unnecessarily high energy of motion which is transformed into a thermal loss that must be dissipated by the collector electrode.

It is an object of the present invention, there- 7 fore, to provide an electron multiplier which may A further object of the invention is to provide a class C operated electron multiplier where the instantaneous voltage difference between the last multiplying stage and the collector electrode is approximately constant during the current conducting period and of such an amplitude that substantially all electrons liberated from the last multiplying stage are attracted by the collector electrode.

In accordance with the present invention there is provided a high efiiciency electron multiplier comprising means for developing an electron stream and means for controlling the electron stream in accordance with an input signal. There are further provided a secondary electron emissive stage operative for multiplying the electron stream by secondary electron emission and a collector electrode arranged to receive the electrons liberated from the multiplying stage. Means are finally provided for impressing an alternating voltage between the multiplying stage and the collector electrode. In this manner the accelerating voltage between the multiplying stage and the collector electrode is maintained at an approximately constant value during the current conducting period of the multiplier, thereby to improve the efliciency of the multiplier when operated as a class C amplifier.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a schematic diagram of an electron multiplier and associated circuits embodying the present invention, while Fig. 2 is a graph showing the steady and instantaneous voltages of the last multiplying stage and of the collector electrode of the multiplier of the invention.

Referring to Fig. 1, there is illustrated schematically an electron multiplier I comprising cathode 2 which may be indirectly heated, as indicated, control grid 3, a plurality of secondary electron emissive stages such as l, 5 and 6 and collector electrode 7 all provided in an evacuated envelope 8. An electromagnetic focusing field may be provided by magnetic coil It energized from a suitable source, such as battery I I.

For the purpose of supplying operating voltages to the electrodes of multiplier I there may be provided a suitable voltage source, such as battery l2, having its terminals connected across voltage divider l3. By means or taps I4, i5 and IE on voltage divider l3 multiplying stages 4 to 6 are maintained at increasing positive potentials with respect to cathode 2 connected to voltage divider 53 through tap l1. Control grid 3 is main tained, by variable tap [8 on voltage divider l3, at a potential that is negative with respect to that of cathode 2.

Collector electrode 1 is maintained at a positive potential with respect to the last multiplying stage 6. To this end there-may be provided a suitable voltage source, such as battery 20, have ing its negative terminal grounded while .its ,positive terminal is connected tocollector electrode I through tuned tank circuit 2|. Battery 20 may be bypassed to ground for alternating currents through bypass condenser 22. Th output signal may be derived from. output coil 23 inductively coupled to tank circuit 2 l.

A suitable input signal which may be developed by signal source 25 is impressed between control grid 3 and cathode 2. The input signal may consist of a carrier wave as is conventional in a class C amplifier. The carrier wave developed across coil 26 is impressed upon tuned circuit 27 provided between control grid 3 and variable tap I8.

Under the influence of the electromagnetic field developed by coil and the electrostatic fields between cathode 2, multiplying stages 4 to 6 and collector electrode '5, the primary electrons developed by cathode 2 impact successive multiplying stages 4, and 6 to liberate secondary electrons therefrom. The secondary electrons liberated from the last multiplying stage 6 are collected by collector electrode I. Electron multiplier I has been shown as a multiplier of the type where the electrons are focused on successive multiplying stages by a combined electrostatic and electromagnetic field. It is to be understood, however, that any conventional multiplier may be substituted for that illustrated by way of example at l in Fig. 1 including a multiplier utilizing only electrostatic fields for focusing the electrons.

Th operation of the electron multiplier as above described is conventional. The multiplier is preferably operated so that the current conducting period is appreciably less than one half a cycle of the carrier wave of the input signal. The current conducting period depends upon the grid bias which may be varied by means of tap I3. The primary electron stream is now multiplied in a conventional manner in successive multiplying stages and collecting by collector electrode i. The current pulses thus developed excite tank circuit 2| which should have a high Q, as is conventional. The instantaneous voltage which is thus impressed upon collector electrode 1 will be of substantially sinusoidal form.

Referring now to Fig. 2, curve 3!! represents the instantaneous voltage of collector electrode 1 of which approximately one half cycle has been shown. The steady voltage of collector electrode 1, which is determined by battery 20, is indicated by line 3|. The current conducting period is indicated by line 3|. The current conducting period is indicated between vertical lines 32 and has been assumed to be 60 degrees. The current conducting angle depends, as explained hereinbefore, on the bias of control grid 3.

According to conventional practice, a steady voltage indicated by dotted line 33 would be impressed upon the last multiplying stage 6 so that at the minimum value of the collector voltage represented by curve 30 the voltage difierence between the last multiplying stag 6 and collector electrode 1 is still suificient for attracting substantially all electrons liberated from the last multiplying stage 6. It will readily be seen from Fig. 2 that at the beginning as well as at the end of the current conducting period the instantaneous voltage difference between the last multiplying stage 6 and collector electrode 1 is considerably larger than required. This higher voltage is converted into thermal energy represent-- ing loss as has been explained'hereinbefore.

In accordance With the present invention this loss in efficiency is substantially eliminated by keeping the instantaneous voltage difference between last multiplying stage 6 and collector electrode 1 approximately constant during the current conducting period. To this end an alternating voltage is impressed between multiplying s'tage'B and collector electrode 1. This alternating voltage has a frequency which equals that of the carrier wave of the input signal developed by signal source 25. The polarity of this alternating voltage is such that during the current conducting period collector electrode 7 is positive with respect to multiplying stage 6 so that substantially all electrons liberated from last multiplying stage 6 are attracted by collector electrode l'. The alternating voltage may be impressed, for example, on last multiplying stage 6 so that its instantaneous value varies as indicated by curve 3 1 in Fig. 2. Furthermore, this alternating voltage should be in phase with the voltage swing of collector electrode 1 so that the alternating voltage impressed on last multiplying stage 6 and the alternating voltage of collector electrode I both reach their minimum value simultaneously. Line 39 shows the steady voltage of last multiplying stage 6.

As shown in Fig. 1 the alternating voltage impressed upon multiplying stage 6 may be derived from signal source 25. To this end coil 35 may be inductively coupled to output coil 26 of signal source 25. Adjustable phase shifter 36 may be provided for the purpose of obtaining an output voltage of the required phase. Amplifier 37 has its output connected to multiplying stage 6 thereby to impress an alternating voltage of the required amplitude thereon. Amplifier 31 includes cathode 38, control grid 40 and anode 4| which is connected to multiplyingstage 6. The signal obtained from, coil 35 at a suitable phase determined by phase shifter 35 is impressed upon control grid 40 having grid leak resistor 42. Cathode 38 may be connected to the anode voltage supply indicated by battery 43. The output voltage is developed across load impedance 44. Amplifier 3'! therefore functions to impress an alternating voltage on multiplying stage 6 as indicated by curve 34 of Fig. 2. A

It Will accordingly-be seenthat the instantaneous voltage difference betweenmultiplying stage 6 and collector electrode 1 is approximately constant during the current conducting period. This will greatly reduce or eliminate the thermal losses of collector electrode'l and hence increases the efiiciency of the multiplier. It will of course be evidentthat acertain ,amountof harmonic distortion of the output wave .will be present. This is due to the fact thatthe peak of the alternating voltage wave is out 01f. However, the resulting distortion of the output signal is of no importance in most applications of a class C amplifier or electron multiplier.

It is to be understood that the alternating voltage impressed upon last multiplying stage 6 may also be derived from the output circuit such as guests from tank circuit 2!. The class C multiplier of the invention may be modulated in any suitable manner as is conventional ina class C amplifier.

' While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

. What'is claimed is: i v

1. A high efficiency electron multiplier comprisingmeans for developing an electron beam, a source of an input signal including a carrier wave, means coupled to said signal source for controlling the intensity of said beam in accordance with said input signal in the manner of a class C amplifier, a plurality of secondary electron emissive stages arranged for multiplying said beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, a tank circuit coupled to said collector electrode to impress on said collector electrode an alternating output voltage, means coupled to said signal source for developing an alternating voltage at the frequency of said carrier Wave, and means coupling said alternating voltage developing means and said last stage for impressing said developed alternating voltage between said collector electrode and said last stage in such a phase as to provide during the current conducting period of said multiplier approximately constant instantaneous voltage difference between said last stage and said collector electrode.

2. A highefiiciency electron multiplier comprising means for developing an electron beam, asource of an input signal including a carrier wave, means coupled to said signal source for controlling the intensity of said beam in accordance with said input signal so that an electron current flows during less than one half cycle of said carri'erwave, a plurality of secondary elecfor impressing said developed alternating voltage between said last stage and ground in such a phase as to provide during the current conducting period of said multiplier an approximately constant instantaneous voltage difference between said last stage and said collector electrode, said voltage difference having an amplitude so that substantially all electrons liberated from said last stage are attracted by said collector electrode.

3. A high efliciency multiplier comprising, means for developing an electron beam, a source of an inputsignal including a carrier wave, means coupled to said signal source for controlling the intensity of said beam in accordance with said input signal so that an electron current flows during less than one-half cycle of said carrier wave, a plurality of secondary electron emissive stages arranged for multiplying said beam, a col- 6 lctor electrode arranged to receive the electrons liberated from the last one of said stages, means coupled to said collector electrode to develop and impress on said collector electrode a first alternating voltage, means including a vacuum tube coupled tosaid last multiplier stage to develop and impress on said last stage a second alternating voltage, and means coupling said vacuum tube to said signal source to vary said second alternating voltage in accordance with signals de-g rived from said signal source in such a manner as to provide during the current conducting period of said multiplier an approximately constant instantaneous voltage difference between said last stage and said collector electrode, said voltage difference having an amplitude so that substantially all electrons liberated from said last stage are attracted by said collector electrode.

4. A high efl'iciency multiplier comprising, means for developing an electron beam, a source of an input signal including a carrier wave, means coupled to said signal source for controlling the intensity of said beam in accordance with said input signal so that an electron current flows during less than one-half cycle of said carrier wave, a plurality of secondary electron emissive stages arranged for multiplying said beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, a resonant circuit coupled 'to said collector electrode to develop and impress on said collector electrode an alternating output voltage, a vacuum tube having an output circuit and an input circuit, means coupling said output circuit between said last multiplier stage and said collector electrode, and means coupling said input circuit to said signal source to impress an alternating voltage derived from said signal source between said collector electrode and said last stage in such a phase as to provide during the current conducting period of said multiplier an approximately constant instantaneous voltage difference between said last stage and said collector electrode, said voltage difference having an amplitude so that substantially all electrons liberated from said last stage are attracted by said collector electrode.

5. A high efficiency multiplier comprising, means for developing an electron beam, a source of an input signal including a carrier Wave, means coupled to said signal source for controlling the intensity of said beam in accordance with said input signal so that an electron current flows during less than one-half cycle of said carrier wave, a plurality of secondary electron emissive stages arranged for multiplying said beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, a tank circuit coupled to said collector electrode to impress on said collector electrode an alternating output voltage, a vacuum tube having a cathode, an anode and a control grid, means coupling said anode and said cathode between said last multiplier stage and said collector electrode, and means coupling said control grid to said signal source whereby to impress an alternating voltage derived from said signal source between said collector electrode and said last stage in such a phase as to provide during the current conducting period of said multiplier an approximately constant instantaneous voltage difference between said last stage and said collector electrode, said voltage difference having an amplitude so that substantially all electrons liberated from said last stage are attracted by said collector electrode.

6. A high efficiency multiplier comprising,

means for developing an electron beam, a source of an input signal including a carrier wave, means coupled to said signal source for controlling the intensity of said beam inaccordance with said input signal so that an electron current flows during less than one-half cycle of said carrier Wave, a plurality of secondary electron emissive stages arranged for multiplying said beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, a tank circuit coupled to said collector electrode to impress on said collector electrode an alternating output voltage, a vacuum tube having a cathode, an anode and a control grid, means coupling said anode to said last multiplier stage, means coupling said cathode to said collector electrode, and means coupling said control grid to said signal source to impress an alternating voltage derived from said signal source between said collector electrode and said last stage in such a phase as to provide during the current conducting period or said mmtiplier an approximately constant instantaneous voltage diiierence between said last stage and said collector electrode, said voltage difierence having an amplitude so that substantially all electrons liberated from said last stage are attracted by said collector electrode.

7; A high e'fliciency multiplier comprising, means for developing an electron beain, a source of an input signal including a carrier wave, means coupled to saidsignal source for controlling the intensity of said beam in accordance with said input. signal so that an electron current flows during less than one-half cycle of said carrier wave, a plurality of secondary electron emissive stages arranged for multiplying said beam, a collector electrode arranged to receive the electrons liberated from the last one of said stages, a tank circuit coupled to said collector electrode to impress on said collector electrode an alternating output voltage, a vacuum tube having a cathode, an anode and a control grid, means coupling said anode to said last multiplier stage, means coupling said cathode to said tank circuit, and means coupling said control grid to saidsi'gnal source to impress an alternating volta ederived from said signal source between said collector electrode and said last stage in such a phase as to provide during the current conducting period of said multiplier an approximately constant instantaneous voltage difference between said last stage and said collector electrode, said voltage difference having an amplitude so that substantially all electrons liberated from said last stage are attracted by said collector electrode. V

8. A high efiiciency multiplier comprising, meansfor developing an electron beam, a source of aninput signalincluding a carrier wave, means coupled to said signal source for controlling the intensity of said beam in accordance with said input signal so that an electron current flows during less than one-half cycle of said carrier Wave, a plurality of secondary electron emissive stages arranged for multiplying said beam, a collector electrode arranged to receive the electrons liberated from the last-one of said stages, a tank circuit coupled to said collector electrode to impress on said collector electrode an alternating output voltage relative to ground, a vacuum tube having a cathode, an anode and a control grid, means coupling said anode to said last multiplier stage, means coupling said cathode to ground, and means including a phase adjusting device coupling said control grid to said signal source to impress an alternating volt age derived from said signal sourcebetween said collector electrode and'said last stage in such a phase as to provide during the current conducting period of said multiplier an approximately constant instantaneous voltage difference between said last stage and said collector electrode, said voltage difference having an amplitude so that substantially all electrons liberated from said last stage are attracted by said 'collector electrode.

JOSEPH C. FERGUSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2 2333% M03919 0 1 19 2,416,376 Cawein Feb. 25, 1947 

